Insulated electrical conductor and method of fabricating the same



Nov. 23, 1948. L. A. BoNDoN INSULATED ELECTRICAL CONDUCTOR AND KETHOD 0FFABRICATING THE SAME Filed April 9. 1947 INVENTOR. Lewis A. Bondon WM LMATTORNEYS Patented Nov. 23, 1948 INSULATED ELECTRICAL CONDUCTOR ANDMETHOD OF FABRICATING THE SAME Lewis A. Bondon, Arlington, N. J.

Application April 9, 1947, Serial No. 740,310

1 claims. l

.This invention relates to insulated electrical conductors and cablesand a method of fabricating them and has more particular reference toinsulated conductorsand cables wherein the insulating covering is formedof laminated layers of insulating material.

Conventional forms of insulating covering for electrical conductors andcables usually comprise a solid, homogeneous layer of insulatingmaterial such as synthetic resin or rubber, or physically reinforceddielectric tapes of various types of insulating material such asasbestos and varnished cambric, which also may be saturated with variousdielectric impregnants. The latter form of insulating coverings areusually fabricated by helically or spirally winding several layers ofvarnished cambric tape about the conductor, after which an outer textilecover is superimposed on the cambric by a braiding or similar operation.In certain types of taped coverings, a layer of felted asbestos isapplied directly to the conductor before being wrapped with thevarnished cambric tape; in other types, additional layers of asbestosare used before the braided covering is applied.

Another form of taped insulating covering cornprises a layer of feltedasbestos applied directly to the conductor; a layer of rubberhydrochloride formed by helically winding a thin tape of rubberhydrochloride around the asbestos; a second layer of elted asbestossuperimposed on the rubber hydrochloride; and an outer covering ofasbestos braid.

All of the foregoing forms of insulating coverings are subject to one ormore of the following objectionable features:

( a) Poor thermal stability.

(b) Poor dielectric performance.

(c) Inability to perform dielectrically in continuous high' temperatureservice without becoming brittle and fracturing upon slight impact orshock.

(e) Lack of flexibility.

(f) Progressive deterioration due to age.

(g) Inflammability of material.

(h) Limited in application to certain types of Wires and cables.

(i) Restricted to low voltage applications.

(7') Unable to perform in humid atmosphere without introducing largevoltage losses.

One object of the present invention is to provide an insulatedelectrical conductor or cable having an insulating covering in which allof the above mentioned objectionable features are overcome.

Another object of the present invention is to provide an insulatedelectrical conductor or cable having an insulating covering which isphysically strong and flexible, possesses high electrical and thermalstability values, and is impervious to moisture.

Another object of the present invention is to provide an insulatedelectrical conductor or cable having an insulating covering generally inthe form of a thin unreinforced dielectric lm or tape of insulatingmaterial helically or spirally wound around the conductor or cable, witheach convolution of the tape overlapping the preceding convolution apredetermined amount, and with a semi-liquid insulating material forminga dielectric seal between the overlapped surfaces of the tape andfilling the open spaces formed at each overlapped edge of the tape,thereby preventing any air from being trapped in the covering.

Another object of the present invention is to provide an insulatedelectrical conductor or cable, as characterized above, wherein the tapeis made of polytetrafluoroethylene and the semi-Huid insulating sealantis of the organo-silicone group.

Another object of the present invention is to provide an improved methodof fabricating an insulated electrical conductor or cable, ascharacterized above.

Another object of the present invention is to provide an insulatedelectrical conductor or cable, as above characterized, wherein thelaminar insulation is enclosed in a braided outer jacket and given acoating of waterproof, Iire resistant material,

A further object of the present invention is to provide a braided outercovering for an insulated conductor or cable which is formed ofinterwoven glass and cotton thread with the cotton thread having alarger diameter than the glass thread, thereby providing improvedphysical confinement and better abrasive resistance.

Other objects and advantages of the invention will become apparent fromthe speccation when considered with the accompanying drawings, wherein:

Fig. 1 illustrates a multiple strand cable embodying the invention;

Fig. 2 is a vertical cross-sectional View taken on the line 2--2 of Fig.1; and

Fig. 3 is a partial longitudinal sectional view, on a slightly enlargedscale, taken on line 3-3 of Fig. 1, with the braid fibres being shown onan exaggerated scale.

In manufacturing insulated conductors or cables of the invention, theconductor element is first coated with a semi-liquid insulatingmaterial, preferably one of the organo-silicone group. Then a thin,fiexible, unrelnforced and initially unstressed film `or tape ofinsulating material, preferably poiytetrafluoroethylene, coated orwetted with the semi-liquid insulating material, is spirally orhelically wrapped, under high tensile stress, around the conductorelement, with each convolution of the tape overlapping the precedingconvolution by a predetermined amount, to form an insulating layer of apredetermined thickness.

The tape is wrapped around the conductor element in a suflicient numberof layers to provide the necessary dielectric strength for theperfoi-mance required. As the tape is tightly wrapped around theconductor element, the semiliquid coating thereon is squeezed out toiill the open spaces formed at each overlapped edge of the tape. Thisprevents the entrapment of any air between the overlapped layers andprovides a thin dielectric film of the semi-liquid or grease between theoverlapped surfaces of the tape, which acts as a lubricant to facilitatethe movement of the overlapped surfaces when the insulated conductor orcable is bent or flexed. After the conductor element has been Wrapped toproduce the required thickness of insulation, a reinforcing, physicallyconnng jacket or outer covering is formed thereon, preferably bybraiding. The jacket may be made of woven cotton or glass fibres;preferably, however, the jacket is made of an interwoven combination ofglass and cotton fibres. After the braid has been applied it isimpregnated with a suitable flame-proof saturant, preferably siliconevarnish, or encased in a silicone rubber jacket and subsequently exposedto a curing and stress relieving heat exposure. The particular heatexposure necessary to stress relieve the fabricated dielectric core mustbe varied because of the temperature limitations of the reinforcingbraid.

Glass braided cores can be relieved in 1.5 to 2.0 hours at 250 C.Cotton-glass combination braided cores can be relieved in 6 to 8 hoursat 150 C.

If the braid is to be finished with well known lacquer formulations, itis necessary that such finishes be applied after the heat treatingprocess, due to the low heat deformation temperature of such syntheticresin finishes.

Polytetrafiuoroethylene possesses an inherent ory. This characteristicwill cause initially mechanically unstressed polytetrauoroethylene formsto return or tend to return to their original form after having beenmechanically stressed in process and then subsequently subjected to astress relieving heat exposure. Full advantage is taken of this physicalcharacteristic in the fabrication of the dielectric core. The initiallyunstressed polytetrauoroethylene tape is maintained under sufcienttension during the wrapping process to cause it to be extended undersuch stress. In addition, the tape, while under such tensile stress willbe subjected to varying degrees of elongation across the tape Width, dueto the various diameters that it lays upon and that are produced by anypredetermined amount of overlapping.

The resultant fabricated dielectric material will thus be under variousdegrees of mechanical stress within a single tape Width and will, upon asubsequent subjection to a stress relieving heat exposure, tend toassume the original unstressed form and in so doing will cause theentire crosssection to shrink radially, thus binding the layers togetheras a solid homogeneous mass. In this process, the organo-siliconesemi-fluid acts as a lubricant and prevents the seizing of the lappedsurfaces, thus permitting uniformly distributed radial confiningpressure throughout the entire dielectric cross-section.

Referring now to the drawing, there is shown, in Fig. i, an insulatedelectrical cable embodying the invention and comprising a conductorelement i0, consisting of a plurality of metallic conductors il aninsulating covering I2 formed on the conductor element; and an outerbraided reinforcing cover member I3, encasing the conductor element andthe insulating covering.

The insulating covering I2 is formed on the conductor element I I byspirally or helically wrapping, under high tensile stress, a thin,ilexible, unreinforced and initially unstressed film or tape lli ofinsulating material around the conductor element, with each convolutionof the tape overlapping the preceding convolution a predeterminedamount. The tape I4 may be of any desired thickness or width, dependingupon the requirements of the particular cable being constructed. In theparticular embodiment shown, the tape is made of polytetrafiuoroethyleneand is .005" thick and l in width. It possesses high tensile strengthand is resilient and flexible. The tape is spirally wound upon theconductor element so that each convolution overlaps the precedingconvolution by three-fourths of its width. When wrapped in this manner,an insulating layer of 300% overlap is formed on the conductor element;in other words, at all points along the conductor element, theinsulating layer is formed of four tapes or an equivalent soliddielectric thickness of three times the tape thickness. As many layersof insulating tape as desired may be applied. As shown in Fig. 1, thetape is wound around the conductor to form two layers indicatedgenerally at I5 and I5, thereby providing a wall insulation having athickness of .040", or .020 wall thickness per layer.

In order toseal any pores which may exist in the polytetrafluoroethylenetape and to exclude all air from within the insulated cable, the outersurface of the conductor is filled with, and the conductor and the tapeare coated with a semiliquid insulating material Il, preferably one ofthe organo-silicone group, prior to the wrapping of the tape around theconductor. This results in the formation of a film of the semi-liquidorgano-silicone material between the surfaces of the conductor and thetape and between the overlapping surfaces of the tape. In addition, asthe tape is tightly wrapped around the conductors, the semi-liquidmaterial is distributed throughout the overlapping surfaces and fillsthe openings formed at each overlapped edge, thereby preventing any airfrom being entrapped within the insulating cover. The semi-liquid orgrease performs an additional-function in acting as a lubricant tofacilitate the sliding movement and minimize mechanical stressing of thedielectric cross-section when the cable is bent or flexed.

After the conductor element has been wrapped with the number of layersnecessary to provide the required dielectric performance which, in themodification shown in Fig. 1, is two layers, a reinforcing, physicalconning jacket or outer covering i2 is formed thereon, preferably bybraiding. The jacket I2 may be made of Woven cotton or glass fibres;preferably, however, and as shown in Fig. 1, the jacket is made of aninterwoven combination of cotton fibres I8 and glass fibres `nated witha suitable flame-proof saturant, prefg erably an organo-siliconevarnish. Then the cable is subjected to a heat of 150 C. for a period offrom 6 to 8 hours for curing purposes vand toy 'stress relieve thedielectric core for the purpose and in the manner hereinbeforedescribed. Organo-silicone varnish on cotton-glass fibre interwovensurface can be rated conservatively at 150 C., while the same varnishonthe glass fibre will permit operation up to250 C.

While a braid made lsolely of cotton fibres or of glass iibres'maybe-used, as above stated, a cotton-glass fibre interwoven braid ispreferable for the following reasons:

1. The glass fibre has high tensile strength and does not yield orstretch, thus maintaining uniform radialconflnement of the dielectriccore, which requires such physical support.

2. 'Ihe proper size cotton fibre, when interwoven with the glass fibre,forms a larger diameter and thus provides abrasion or scu protection tothe interwoven glass libres. `The glass fibres alone are unsatisfactoryphysically, because they will on continuous flexing, fracture at thepoints of crossing in the braid, where they are subjected to their ownsawing action. 3. Upon exposure to flame or intense heat, cotton fibreswill disintegrate and, if it were not for the remaining confining glassfibres, the dielechaving been mechanically stressed in process and thensubsequently subjected to a stress relieving heat exposure.

v 3. The organo-silicone iluid or grease has excellent wettingproperties and will uniformlxn wet the surfaces ofthepolytetrafluoroethylene tapes.

C. Electrically 1. Polytetrafiuoroethylene dielectrically is a superiorinsulating material and has .electrical properties which enable it toprovide improved given conductor size produces a smaller diameter withimproved dielectric strength. This feature is advantageous for all wiresand cables and especially for wires and cables used in aircraftapplitric core would be unsupported and fail electrically. With thecombination cotton and glass fibre braid, the wire is able to supplyservice after flame exposures which would render conventional wireuseless.

Cables and conductors provided with an insulating core employingpolytetrafiuoroethylene and an organo-silicone grease or semi-liquid asdielectrics and fabricated as above described, will have superiorthermal, physical, electrical and chemical qualities and will be freefrom all of the objectionable features as above pointed out which areinherent ina more or less degree in all conventional types of cables andconductors. This is due to the particular construction of the insulatingcore and the particular characteristics of the dielectrics employed,which are as follows:

A. Thermally l. Polytetraluoroethylene as a solid is unaffected by heatfrom -70 C. to 300 C.

2. Organo-silicone compound is heat stable from 70 C. to 250 C. 3.Neither of these materials will carbonize upon flame exposure.

B. Physically i. Polytetrafluoroethylene in conventional solid extrudedform on a wire is a very rigid member cation where light weight isessential.

(b) Power factor or loss angle is extremely low and is stable over awide frequency range, thereby permitting the use ofpolytetrafluoroethylene in all types of wires and cables forinstruments, heater cords, low-tension, high tension, ignition`transformer leads, coaxial, heater elements, high tension aircraftignition terminal disconnect seals for very high and low temperatureapplication, etc.

(c) Dielectric constant-Has extremely lowv specific inductive capacityvalue and can be used where other types of insulation would beobjectionable, due to their higher capacities.

(d) Insulation resistance-Extremely high.

2. Organo-silicone compound:

(a) Has dielectric properties which improve the performance ofpolytetrailuoroethylene in this laminated construction by excluding airocclusion from the conductor surface and in-between the lapped surfaces.

(b) Wets polytetrafluoroethylene surfaces and reduces the danger ofporosity effect of the polytetrafluoroethylene which, alone, would causethe cable to fail, due to premature local ionization and flashover. Thisporosity accompanies all polytetrafiuoroethylene tapes and solidextruded forms to a certain degree and will result in unstabledielectric performance. Therefore, with this dielectric combinationthere is improved dielectric stability and a resultant increase incorona initiation voltage level.

(c) High insulation resistance which does not vary to any great degreewith prolonged heat or humidity exposure.

D. Chemically l. Polytetrailuoroethylene and organo-silicone compoundsare unaffected by chemical or metallic reactions and arenot affected bycorona which may form about the cable in operation. Both dielectrics incombination, therefore, act together to form a barrier tc preventmoisture or conductive gases from entering into the dielectric core.which normally would cause reduced insulation resistance and alsosubsequent ilashover along the moist surfaces. l

2. This dielectric combination will not carbon ize as do conventionalorganic insulations which renders a cable inoperative due to theresultant carbon track formed.

From the foregoing, it will be seen that there has been provided animproved insulated construction for wires and cables which has thefollowing advantages over the present conventional type insulated wiresand cables, in addition to those hereinbefore mentioned:

A. Higher current rating The dielectric combination of the presentinvention is rated'from 70 C. to 25 C. Therefore, for a given conductorsize, increased ratings beyond the present conventional vtypes areapparent. l

B. Flame proof Both poiytetrailuorcethylenc and Iorgano-siliconecompounds are flame prooi.

C. Reduced diameters for given size conductors The high dielectricstrength of the dielectric combination, which is unailected by prolongedaging, will permit reduced diameters for given size conductors.

D. High insulation resistance Because of the nature and construction ofthis dielectric combination, insulation resistance values are well abovethose of conventional dielectrics in wire applications and will remainintact over long periods of high humidity exposures as well ascontinuous submersion inwater.

The dielectrics in combination are both nonhygroscopic and will not takeup moisture within its own mass. However, if used alone, thepolytetrailuoroethylene tapes with conventional liabrication with theshort overlapped surface will pick up moisture after afshort applicationto ilexing and bending stresses which would extend and compress the tapeat the stressed areas and develop laminar porosity. While the degree ofthis porosity is minute, it is detrimental to the insulation resistanceas well as other electrical properties. However, when insulating wallsare applied in combination as disclosed, it eliminates this possibilityas the organo-silicone lubricant wets and seals the tightly laminatedsurfaces.

E. High dielectric strength When compared with currently known tapes andtheir lubricating agents (usually e, petroleum derivative) thiscombination exceeds their performance dlelectrically, thermally andphysically, and it is only necessary to apply this Vdisclosedcombination in thin wall insldation to achieve superior performance.

Because of the high tensile strength of polytetraluoroethylene tape, asWell as its ability to yield to the laminated profile in process, it ispos.. sible to overlap itself many times to provide a laminar dielectricwall of a number of times the tape thickness in one taping operation,which is not possible with conventional physically reinforced dielectrictapes. Flexibility is provided by the organo-silicone compound whichwipes each. lap and excludes minute air particles to provide a solidmass in the finished combination dielectric cross-section.

The iinished cross-section is electrically proper from the standpoint ofdielectric placement which requires thepositioning of high specicinductive capacity materials to be adj acent to the conductor surfacesto avoid distress to the other dielectrics in the cross-section in orderthat they may be used most eiectively. The semi-huid organosiliconecompound has a specific inductive capacity value of 2.4i, whilepolytetrafluoroethylene has a specific inductive capacity value of 2.0,

F. Higherv corona forming voltages Due to the exclusion of air from thecross-sec- 'tion of this construction mainly along the conprovided thanthat which exists inthe solid in sulated polytetrauoroethylene wires.This is wires cannot be processed to guarantee uniform dielectricstrength. They cannot be extruded in small diameter or to provide thinwall insulation and they have non-uniform wall thickness. The improvedconstruction of the present invention provides the solution to all theabove diilculties without sacrificing any of the good properties of thepolytetrafluoroethylene but rather improving its performance physicallyand electrically. The resultant flexibility reduces fatiguing efectswhich tend to reorient the originally stress-relieved dielectricmolecular pattern.

All the materials proposed in this construction, as well as the finishedreinforcing braid are not sensitive to the action of corona and will,therelore, perform eiliclently under prolonged corona exposure.

G. High heat stability Because of the extreme temperature limits of allthe materials used in this construction, it is possible to provide awire which will perform beyond temperatures presently permitted by theNational Board of Fire Underwriters.

The highest current rating presently assigned to approved asbestos typesis specified for dry location only and is considerably below thatpossible ln the proposed construction.

Conventional varnish cambric types are limited in temperatureperformance and will, after prolonged exposure .at allowabletemperature, become brittle and fracture on slight impact or shock. Suchcables, when exposed to various sealing compounds in which they areimmersed, will rea-ct chemically and often fail dielectrically aftercuring process.

Conventional synthetic resin or rubber insulated types are -aiected bysoldering operationA Imperviousness to humidity and liquid Tests haveshown that this dielectric combination, which is totally inertchemically and nonhygroscopic, is not affected dielectrically byprolonged immersion in liquid or exposure to highly humid or reactiveatmospheres.

With a lacquered iinished interwoven cottonglass libre jacket, which issensitive to mild alkalis and acid solutions, the dielectric performanceof the core combination remains intact even though the surface finishand the cotton nbre may disintegrate.

The insulating covering of the present invention may be used for alltypes of wire service and, by applying the number of proper gauge tapesin suiicient layers, with proper degree of overlap, will satisfy anyinsulating wall dimension that may be specied and will provide accurate.diametric concentricity.

While in the particular modiilcation of the invention illustrated, theconductor element has been shown as a seven strand cable, obviously, theconductor element may be of any type such as single conductor, multiplestrand conductor or concentric conductors.

Having thus described the invention, what is claimed is:

1. An insulated electrical conductor comprising a metallicconductorelement insulated by a surrounding layer of laminatedpolytetrafiuoroethylene having a film of dielectric lubricant interposedbetween the contacting surfaces of the 1aminatedpolytetrafluoroethylene.

2. An insulated electrical conductor comprising a metallic conductorelement insulated by an insulating layer of high dielectric strength andcomposed of a thin tape of polytetrauoroethylene wound helically aboutthe conductor element, with each convolution of the tape overlapping thepreceding convolution by a. predetermined amount, and with a thin illxnof d-ielectric lubricant coating the surfaces of the conductor elementand the tape.

3. An insulated electrical conductor comprising a metallic conductorelement insulated by a surrounding layer of laminatedpolytetrailuoroethylene having a th'n film of organo-silicone compoundinterposed between the contacting surfaces of the laminatedpolytetrauoroethylene.

4. An insulated electrical conductor as set forth in claim 1, includinga protecting confining covering fabricated from cotton and glass nbreswith the cotton bres being of a larger diameter than the glass fibres toprovide an outer circumferential surface of cotton fibres.

5. An insulated electrical conductor comprising a metallic conductorelement insulated by an insulating layer of high dielectric strength andcomposed oi' a thin tape of polytetrailuoroethylene wound hellcallyabout the conductor element, with each convolution of the tapeoverlapping the preceding convolution by a predetermined amount, andwith a thin nlm of an organo-silicone compound coating the surface orthe conductor element and the tape.

6. An insulated electrical conductor as set forth in claim 5, includinga protecting conning covering of braided cotton and glass' fibresimpregnated with an organo-silicone compound.

7. A process of fabricating insulated electrical conductors or the like,characterized by helically wrapping. a thin, flexible, unreinforced andinitially unstressed tape in the form of a 111m ofpolytetrafiuoroethylene about a. conductor element under suiiicienttension to cause each convolution of the tape to tightly conform to thesurface'about which it is wrapped and with each con# volution of thetape overlapping Ithe preceding convolution by at least three-fourths ofits width to form a laminated ldielectric core; coating the outervsurface of the conductor element and said tape? with an organo-siliconesemi-liquid prior to wrapping said tape thereon to prevent any air frombeing entrapped within the dielectric core and -to lubricate theoverlapping convolutions; braiding a reinforcing, physically confiningouter jacket on said core; and subjecting the complete assembly -tosuicient heat to stress relieve the laminated core.

LEWIS A. BONDON.

REFERENCES CITED The following references are of record in the ille ofthis patent:

v UNITED STATES PATENTS Number Name Date 232,122 Hammesfahr Sept. 14,1880 1,749,740 Frederickson Mar. 4, 1930 2,004,004 Knoderer June 4, 19352,164,904 Cook July 4, 1939 2,258,218 Rochow Oct. 7, 1941 2,335,088Shoemaker Nov. 23, 1943 2,392,388 Joyce Jan. 8, 1946

